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International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19, 2009

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Organized by State Key Laboratory of Lithospheric Evolution Institute of Geology and Geophysics, CAS Xinjiang 305 Project

International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19,2009

Permian-Triassic transcontinental shear zones in Southern Mongolia Boris A. Natal'in 1, Tomurhuu Dondov 2, Ariunchimeg Yarimpil 2 , and Otgonbaatar Dorjsuren 2 1 - Istanbul Technical University, Istanbul, Turkey, natalin(mitu.edu.tr ; 2 - Institute of Geology and Mineral Resources, Ulaanbaatar, Mongolia, dt(a1igmr.mas.ac.mn

During

the

Permian

(270-250

Ma),

the

2000 km

(~eng6r

and Natal'in, 1996). In Altay, a

evolution of the Altaids was controlled by the

foliation produced by this shearing yields 271 Ma

sinistral Irtysh-Gomostaev and dextral Scytho-

Ar-Ar age. In the Triassic, this shear zone was

Turanian shear zones (Fig.

I). The Irtysh-

reactivated with dextral sense of motions as it is

Gomostaev shear zone can be traced as the NW-

evidenced from en-echelon pattem of rifts and

trending narrow zone stretching from Chinese

grabens in the basement of the West Siberian

Altay to the Polar Urals where it cuts the

Basin (Natalin and 2006).

structures of the Ural ides. The displacement along the Irtysh-Gomostaev shear zone was estimated as

~eng6r,

2000; Allen et al.

Early-Permian - Triassic 250-210 Ma

[===~=l

Baykalides

~uralides

k>,~~j

Altaids

~ Manchurides

cratons

continental blocks extensional basins The Permo-Triassic Silk Road arc

Fig. 1 Transcontinental shear zones in Northem Asia during the Permian-Triassic (modified after Natal'in and

~eng6r,

2005).

The E-W trending Scytho-Turanian shear zone

of the Russian craton. In the west, its structure and

stretches from Beishan to the southeastem margin

kinematic history are poorly known because of a 44


wide extent of younger sediments. In Western

displacement distr ibuted across a rather wide zone

Tien Shan, the motion along this zone produced

in the northern wall of the Scytho-Turanian Fault.

the Tul melange exposed in the northern slope of

In Mongolia, the Irtysh-Gornostaev and Scytho-

the Alay Range. Contrary to subduction-related

Turanian shear zones join each other making a

melanges the Tul melange contains Devonian to

complicated pattern of branching E-W-trending

Lower

of vanous

faults. Kinematic features of those lying on the

tectonic setting incorporated in a Late Silurian

continuation of the Irtysh-Gornostaev shear zone

matrix.

criteria of steep dipping

were inferred as sinistral from the kinematic

foliation of this matrix indicate dextral sense of

history of the Altaids and Manchurides (Sengor

shear. Farther to the east, near Vuadil Village, this

and Natal' in, 1996) however direct structural

fault zone forms a wide (2-4 km) belt of high-

observations supporting this inference appeared

strain rocks with steep dipping foliation and

later and only in a few locations (Badarch et ai,

horizontal stretching lineation. Late Carboniferous

2002). Our recent studies 30 km south of Hurmen

conglomerates deposited after the collision of the

(H in Fig.2), indicate that Upper Permian clastic

Alay microcontinent are stretched parallel with

rocks form a system of large scale (3 km)

that lineation. This imposes a lower time limit for

asymmetric folds with steep plunging hinges

the ductile dextral shear. The Permo-Triassic age

indicating dextral shearing along NWW-SEE

of faulting can be inferred from synchronous

strikes. This shearing was observed at the northern

dextral displacements along the Talas-Fergana

boundary of the Dzolen tectonic unit representing

Fault (Burtman, 1973). Similar to dextral Junggar

north-facing accretionary wedge paired with the

Fault it can be considered as Reidel shear to the

Paleozoic South Gobi arc (Fig. 2). Post-Permian

Scytho-Turanian Fault.

timing of this dextral shearing is synchronous with

Carboniferous Kinematic

inclusions

In the Chinese Tien Shan, the Central Tien Shan

the above mentioned reactivation of the Irtysh-

Fault is a segment of the Scytho-Turanian Fault.

Gornostaev shear zones and suggests its extent

There, the dextral ductile shearing yields 290 to

across Mongolia.

245 Ma Ar-Ar ages (Laurent-Charvet et aI., 2003).

Tectonic units of Southeastern Mongolia usualy

Recent paleomagnetic studies (Van der Voo et

defined as terranes despite the fact that many of

al.

2006)

have

40-90°

them reveal a common stratigraphic and magmatic

counterclockwise rotation of Permian declinations

record (Fig. 2). Among them there are fragments

in the southern limb of the Kazakhstan orocline.

of Neoproterozoic structures intruded by granites

The similarity of declination orientations in the

of 950 and 916 Ma and covered by Vendian-

northern limb of the orocline with the Baltica

Lower Cambrian carbonates and quartzites. These

reference frame and minor differences of Tarim

fragments are intruded by Silurian to Permian

declinations comparing with Baltica, preclude an

granites preserving roof pendants of arc volcanic

interpretation the orocline formation due to NS

rocks of the same age. Sengor and Natal'in (1996).

shortening (e.g. Alexeiev. 2008). Shearing along

interpret them as a back-stop and single magmatic

the Scytho-Turanian Fault is more plausible

arc

explanation.

of paleomagnetic

Neoproterozoic basement of this unit is also

declinations requires about 1000 km of dextral

known as Tsagaan Uul and Hatag Uul terranes

The

established

rotation

45

defined

as

the

South

Gobi

unit.

The


•

EB

o .

O._.:::30-=60_ _1:roi::=:=::i180 __ 2~""'t.'"

South Gobi Unit - PCm back-stop and Paleozoic arc Hegeshan·Lugingol • south·facing accretionary wedge of the South Gobi Unit Solonker Zone - in the north. it is southfacing accretionary wedge of the South Gobi Unit

~ Dzolen - north-facing accretionary "" wedge of the South Gobi Unit

~ Gobi-Allay

and Gurvansaihan -

~ south-facing arc paired with

Tuva-Mongol ~ Hangai-Hantei Paleozoic ~ accretionary wedge

!]TIl Manchurides - Paleozoic north-

I.LW factng arc of the North China block -

major faults (dominantly strike-slip faulls) in Sou!hem Mongolia

Fig. 2. Tectonic structure of Southern and Inner Mongolia. The Permian-Early Triassic Solonker suture zone was formed due to the northward and southward subduction of the Solonker ocean lithosphere beneath the Paleozoic South Gobi and Manchuride arcs respectively. and overlying the

Silurian~Permian

as Hashaat/Atasbogd and

arc is defined

Carboniferous granites abundant in the arc massif

Enshoo/Nuhetdavaa

fragments (even small ones!) never cut generally

eastern

amagmatic the Devonian and Permian flysch.

continuation of the arc is known as the Uliastai

Permian volcanic rocks constituting the arc

active continental margin. We should emphasize

fragments totally disappear across fault boundaries

that the Neoproterozoic basement is intruded by

with Lugingol Formation. In recent terrane-based

granites and overlain by volcanic rocks of the

studies the Lugingol type rocks are interpreted as

same age as those in the neighboring so-called

an overlap assemblage. Strong deformation of the

"arc

Permian and Devonian flysch, the similarity of

terranes.

In

Chinese

back-arc

territory

terranes".

the

Thus,

the

only

difference between "terranes" is a level of erosion

their structural

and the only reason of "terrane' recognition is

inclusions consisting of Upper Carboniferous-

fragmentation the single arc by long and straight

Lower Permian shallow-marine limestones and

faults. Another rock assemblage consists of thick

Lower Carboniferous arc volcanics preclude this

turbidites that is commonly defined as the

interpretation

Ligingol flysch of Permian age. Mongolian

depositional settings if not subduction-accretion

geological maps also show Devonian flysch in the

origin of the flysch.

same zones sometimes along-strike continuations of the

Lugingol

Formation.

Devonian

styles, existence of tectonic

and

indicate

at

least

forearc

In Inner Mongolia, the northern belt of the

and

Lugingol type rocks continue in the Hegenshan 46


Mountains where ophiolites tectonically mixed

cleavage/bedding intersections, and wide zones of

with Carboniferous and Lower Permian rocks (Fig.

quartz veins showing en-echelon arrangements.

2). Many researches now agree on subduction-

Kinematic criteria indicate sinistral sense of shear.

accretion nature of this mixture. The southern belt

The same structural style has been observed along

of the Lugingol type rocks of Southern Mongolia

the Mongolian segment of the Solonker suture.

continue to the east as the Solonker suture zone

The melange along it includes blocks of oceanic

(Sengor and Natal'in, 1996). Between these two

and magmatic arc origin, fragments of late

belts there is a narrow strip of Ordovician to

Paleozoic

Permian granitic and volcanic rocks originally

Neoproterozoic

defined as the Sonidzuoqi microcontinent (Fig. 2,

embedded in sheared matrix (Lugingol flysch)

Hsu at al. 1991) and later renamed as the in the

with steep foliation. Observed contacts of the

Baolidao arc. Similar to Southern Mongolia old

inclusions and matrix are

xenocryst ages in Paleozoic granites indicate a

contrary earlier claims for a dominance of nappe

presence of the Precambrian basement.

tectonic

carbonate

styles.

Cambrian

Along the northern boundary of the Sonidzuoqi

platform,

basement.

All

Inclusions

marbles

and

and

of them

universally of the

quartzites

are

steep

Vendianand

~

the

late

unit, sinistral shear zones with steep dipping

Paleozoic carbonates (even large ones

foliation and subhorizontal stretching lineations

have a boudin-like shape with crosscutting

have been observed during the IGCP-480 field

relationships of the internal fabric and boudin

excursion in 2007 (location 3 in Fig. 2). Several

margins. Regardless of the validity of age

zones of shearing confined to the margins of the

determinations these structural features show

unit are shown in the Geological map of Inner

strong shearing. Kinematic criteria indicate that

Mongolia (Liqiao Zhang, 1982) however the

the melange formation have to be explained by

kinematics of this shearing remains unknown.

arc-parallel

Taking into consideration the narrowness of the

subduction or "terrane" accretion.

strike-slip

faulting

20x4 km)

rather

then

Sonidzuoqi unit, the similarity of its arc-related

In the central part of Sothern Mongolia (2 in Fig.

magmatic record with those in Southern Mongolia,

2), a wide shear zone separates the basement of

and following Sengor and Natal'in (1996) we

the South Gobi unit (Tsagaan Uul cratonic

interpret the Sonidzuoqi unit as a strike-slip sliver

"terrane" of Badarch (2005)) and the Lugingol-

of the Uliastai active continental margin. This

Hegenshan unit. The structure of this junction has

sliver entraps a part of the Uliastai forearc region,

a similar structural style as in the Solonker suture

the Lugingol-Hegenshan unit, which originally

zone. The only difference is that melange along

was to the south of it. If this inference is correct

this junction was created by the dextral shearing

the sinistral displacement of the Sonidzuoqi unit

synchronous with

should be more than 600 km.

amphibolite facies metamorphism. This shearing

upper

greenschist -

low

In the eastern part of Sothern Mongolia (1 in

affects rocks as young as the Permian. Timing and

Fig. 2), the steep dipping early Permian Lugingol

kinematics of this shearing better fit those along

flysch also reveals bedding parallel shearing

the Scytho-Turanian Fault, particularly along its

associated with asymmetric steep plunging folds,

segment - the Central Tien Shan Fault. The

NE-striking

sinistral strike-parallel shearing was also observed

cleavage

with

steep

plunging 47

International Field Excursion and Workshop on Tectonic Evolution and Crustal Structure of the Paleozoic Chinese Tianshan Urumqi, China, September 9-19,2009

but only as overprir.ting structures showing a

Reference

prolong transpression during the evolution of this region. Near the contact with cratonic unit, the

Allen, M.B., Anderson, L., Searle, R.C. and

Lugingol Formation consists of slates and thin

Buslov, M., 2006. Oblique rift geometry of the

bedded ,turbidites showing i that evidently a wide

West Siberian Basin: tectonic setting for the

zone of transitional facies was eliminated due to

Siberian flood basalts. Journal of the Geological

strike-slip faulting.

Society, London, 163: 901-904.

General structure of the South Gobi unit both in

Natal'in, B.A. and Sengor, A.M.C., 2005. Late

Mongolia and Inner Mongolia may be defined as a

Palaeozoic to Triassic evolution of the Turan and

mega-melange created because of the strike-slip

Scythian platforms: The pre-history of the Palaeo-

shearing. We infer that the mega-melange was

Tethyan closure. Tectonophysics, 404(3-4): 175-

formed during oblique subduction leading to arc-

202. Sengor, A.M.e. and Natal'in, B.A., 1996.

shaving faulting (Natalin and Sengor, 2005) and the

Palaeotectonics of Asia: Fragments of a synthesis.

Solonker suture. Our interpretation contradicts

In: A. Yin and M. Harrison (Editors), The

popular ideas

Paleozoic-early

Tectonic Evolution of Asia. Rubey Colloquium,

Mesozoic tectonic evolution of the region is

Cambridge University Press, Cambridge, pp. 486-

compared with modern tectonic setting of South-

640.

the

following

oblique in

collision

which the

across

Travin,

Eastern Asia. These ideas employing multiple

A.V.

et

aI.,

2001.

40Ar/39Ar

collisions of the above mentioned terranes do not

datirovanie plasticheskikh deformatsii v Irtyshskoi

account for scales of tectonic zones which are

sdvigivoi

active now. For instance, if widths of arc-forearc

(40Ar/39Ar dating of ductile deformation in the

regions in modern arc-trench systems usually

Irtysh strike-slip fault zone (Eastern Kazakhstan)).

exceed 250 km, the widths of reconstructed

Geokhimiya(12): 1-5.

zone

(Vostochnyi

Kazakhstan)

Paleozoic arcs (including microcontinents) can be

Van der Voo, R., Levashova, N.M., Skrinnik,

only 20 to 50 km. Interestingly, descriptions of

L.I., Kara, T.V. and Bazhenov, M.L., 2006. Late

rocks

and

orogenic, large-scale rotations in the Tien Shan

"microcontinents" do not imply even a medium

and adjacent mobile belts in Kyrgyzstan and

strain.

Kazakhstan. Tectonophysics, 426(3-4): 335-360.

constituting

these

"micro-arcs"

48